Refactor docker setup

[Iteron-dev]

Resolves #126 and replaces the abandoned PR #134.

• Moved environment variables from the Dockerfile to docker-compose.
• Improved sandbox fetching:
  • Sandboxes are now cached by Docker.
  • Adding sandboxes to the build is optional (target: development).
  • Caching is based on a manifest, so if the manifest hasn't changed, sandboxes will remain in the cache. This reduces the time required to fetch them by approximately 30 seconds.

Currently, this is a draft because I still need to add GitHub Actions for building the Docker image.

[Iteron-dev]

Added GitHub Actions for building the Docker image.